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Thermomechanical fatigue of SnAgCu/Cu solder joints subjected to thermal cycling was investigated. Based upon an analysis of displacements for flip-chip solder joints subjected to temperature cycling, a special bimetallic loading frame with single solder joints has been designed, which allows for the strain measurements of an individual solder joint during thermal cycling. The strain-stress relationship for the solder joint, characterized by hysteresis loops, was determined by strain gauge measurement during thermal cycling from –40 to 125 oC. The failure process was characterized by resistance change of the solder joints, and the number of cycles to failure was determined when the variable D=1-R0/R approximately reached 0.5. The results show that the failure process of solder joints is accelerated with the increase of strain range applied. Coffin-Masson law was applied to characterize the relationship between solder joint life and inelastic strain range, and the model parameters were determined for SnAgCu/Cu solder joint. In addition, the microstructure evolution of the solder joints during thermal cycling was analyzed by Scanning Electron Microscopy, which gave the microscopic explanation for the failure mechanism of SnAgCu/Cu solder joints.